Strange mesh behavior with proportional edit
LukeD was right, following his tips the "rip" seemed to happen along the local Y axis of the object, moving the mesh in Edit Mode along the X axis so it would no longer overlap the Y=0 axis, would alleviate the problem.

The problem it seems is the Mirror X option that was active in the object editing options. In the 3D View Tools shelf > Options Tab > Mesh Options disable X Mirror option.



There did seem to be some sort of symmetry in the works, I searched for the setting but could not find it anywhere myself, this video however helped shed some light on the issue.